The present invention relates to a toothed belt for power transmission.
Generally speaking, a toothed belt is constructed, as exemplified by toothed belt 1 in FIG. 1, such that a rubber body 2 has teeth 3 formed on one surface thereof and tensile cords 4 buried therein in its longitudinal direction. A woven fabric 5 is adhered along the tooth-shaped contour to the meshing face of the toothed belt 1.
When this toothed belt 1 is to be manufactured, on the outer circumference of a cylindrical mold 7 formed on its surface with a female mold 6 for the teeth 3, as shown in FIG. 2, there is fitted the cylindrical woven fabric 5 which has been subjected to a treatment with an adhesive, e.g., RFL (i.e., Resoroinol-Formaldehyde-Latex). On this woven fabric 5, there are turned the tensile cords 4 which in turn are covered with rubber 8. When this rubber 8 is heated under pressure, it is forced through the gaps between the tensile cords 4 into the female mold 6, as shown in FIG. 3, so that it is vulcanized into the teeth 3. At this time, the woven fabric 5 is forced together with the rubber 8 into the female mold 6 to form a meshing face along the surfaces of the teeth 3. The woven fabric 5 is required to have initially a length substantially equal to the circumferential length of the toothed belt 1 and, when the teeth 3 are to be formed, a length contouring the teeth of the toothed belt 1. For this requirement to be met, it is necessary that the woven fabric 5 can be easily extended in the longitudinal direction of the toothed belt 1.
In the prior art, as the yarns extensible in the longitudinal direction of the toothed belt 1 stretchable yarns have been used. These yarns will be called hereinafter the "longitudinal yarns 9" of the belt lengthwise direction, whereas the yarns to be combined with the longitudinal yarns 9 will be called the "transverse yarns 10". However, these "longitudinal yarns" and the "transverse yarns" are not necessarily coincident with the terminology "warp" and "weft", as used in the textile structure.
These stretch yarns are exemplified by the woolly processed yarns of nylon or the like, as disclosed in JP, U, S 62-2204059, U.S. Pat. 4,826,472 or European Patent No. 271,587. In recent years, toothed belts are used for driving the camshaft or balancer shaft of an automotive engine, for example. In accordance with the rise in engine performance of recent years, the engine room is subject to high temperatures so that the toothed belt is required to have a sufficient durability even in such a hot atmosphere.
In these years, therefore, it has been examined to employ high tensile strength and heat resistance fiber yarns such as aromatic polyamide fiber yarns as the longitudinal yarns 9 of the woven fabric 5 in the toothed belt.
Generally speaking, however, the high tensile strength and heat resistance fiber yarns are difficult to woolly-process and to have a sufficient stretch. In JP, Y, S63-15628, therefore, it is disclosed to employ as the longitudinal yarns 9 of the woven fabric 5 of the toothed belt 1 the covered yarns 13 each of which has a polyurethane elastic yarn 11 spirally covered with an aramid fiber yarn 12.
As the polyurethane elastic yarn 11 shrinks, the spiral aramid fiber yarn 12 slackens so that the covered yarn 13 can stretch by a length corresponding to the slack. In this covered yarn 13, however, the polyurethane elastic yarn 11 is covered with a sufficiently long aramid fiber yarn 12 so that no mutually restraining force acts between the polyurethane elastic yarn 11 and the aramid fiber yarn 12. As a result, a slip occurs in-between so that the slack of the aramid fiber yarn 12 appears in the form of irregular loops 14 as shown in FIG. 4. If the covered yarn 13 is woven as the longitudinal yarns 9 into the woven fabric 5, the loops 14 protrude from the surface of the woven fabric 5 to make the woven fabric 5 bulky.
In view for the purpose of using the toothed belt 1, the tensile cords 4 are preferably disposed as close to the bottoms of the teeth 3 as possible, so that the woven fabric 5 may be prevented from growing bulky. If, moreover, a number of loops 14 irregularly protrude, they deteriorate appearance of the toothed belt. Still the worse, numerous loops 14 are liable to be worn locally at the bottoms by the contact with the teeth of the pulley so that the toothed belt 1 has its durability deteriorated.
In the aforementioned covered yarn 13, moreover, the aramid fiber yarn 12 is exposed at the outermost portion of the covered yarn 13. As a result, the aramid fiber yarns 12, the rubber of the rubber body 2 and the teeth 3 are directly adhered into an integral structure, in which the aramid fiber yarns 12 are restrained by the rubber to have its degree of freedom lowered. As a result, if the toothed belt 1 is subjected to repeated deformations, the aramid fiber yarns 12 are concomitantly subjected to repeated extensions and bendings whereby their strength lowered due to fatigue thereby causing a serious deterioration in the durability of the toothed belt 1.
The present invention has been accomplished in view of the background thus far described and has the object of providing a toothed belt which is excellent not only in heat resistance but also in wear resistance and durability.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.